wall unit vs piece of furniture

wall unit and piece of furniture both are nouns.

Word	Noun	Adjective	Verb	Adverb
wall unit	Yes	No	No	No
piece of furniture	Yes	No	No	No

As nouns, piece of furniture is a hypernym of wall unit; that is, piece of furniture is a word with a broader meaning than wall unit:

wall unit: a piece of furniture having several units that stands against one wall of a room
piece of furniture: furnishings that make a room or other area ready for occupancy

Other hypernyms of wall unit include article of furniture, furniture.

wall unit (noun)	piece of furniture (noun)
a piece of furniture having several units that stands against one wall of a room	furnishings that make a room or other area ready for occupancy